How would the rear diff fail unless run without oil? CVs only seem to fail if a boot rips, even then it will still run for a long time without lubrication. Periodic inspection will prevent this. I have 317k on a fully stock 722.351 in my 300SEL.
There is a specific tool so you can depress the dual springs on the M116. Works well. You remove the rocker first, then the springs. I would not waste my time again with stem seals, but it could buy you some time. They did not do much for me.
From what I was told by a tech, the angle of the rocker being depressed puts a side load on the guide causing them to ovate some...hence the excessive wear. My machinist says pretty much all the SOHC M116/7s need guides past 100k. You and I are both long over due.
Here is the tool -
Mercedes Valve Spring Compressor | eBay